Hearing restoration or compensation devices, commonly known as hearing aids, provide a tremendous benefit to a patient with congenital hearing loss or whose hearing has deteriorated due to age, genetics, illness, or injury. There is a wide variety of commercially available devices that can be worn externally or can be implanted within the body of the patient.
In an implantable hearing aid or other implantable medical device, it is generally important to know the status of the battery charge. If the battery were to drain beyond a particular threshold, it might lead to a failure of the device. For a hearing aid, such a failure would result in silence for the user. For other medical devices, a loss in service might have more severe consequences. As a result, many implantable devices include a mechanism to alert the user when the battery charge is low.
One of the simplest alert mechanisms uses a measure of the battery voltage. If the battery voltage dips below a predetermined threshold, then the user is alerted. However, this simple alert mechanism works only if the battery voltage decays as the battery is drained. For many implantable medical devices, the battery is designed to maintain a nearly flat voltage over the lifetime or the cycle of the battery. This flat voltage profile typically simplifies the design of the internal electronics, but usually prevents using the battery voltage itself to determine the remaining charge in the battery.
Another alert mechanism, which does not rely on the battery voltage directly, uses a circuit commonly known as a “charge integrator” or a “coulomb counter”. For a simple coulomb counter, a resistor is placed in series with one of the battery terminals, and current flowing through the resistor is inferred by measuring the voltage across the resistor. The coulomb counter circuit acts like an integrator, so that the voltage across the resistor is measured over time and therefore represents a quantity that is proportional to a time integral of the current flowing through the resistor. Such a time-integrated current is the cumulative electrical charge that has flowed through the resistor. One can then compare the cumulative electrical charge detected by the circuit with the fully-charged capacity of the battery, and alert the user when the percentage of capacity reaches a particular threshold.
The simple coulomb counter runs into difficulty for devices that have a wide dynamic range of operating current, such as implantable hearing aids. The output power of an implantable hearing aid varies with the ambient sound environment around a user, and can vary over a dynamic range between 60 dB and 80 dB. Specifically, the problem with such a large dynamic range arises from the resistor that is in series with one of the battery terminals. If a resistance value is chosen to accommodate large currents (corresponding to loud volumes), then at low volumes the voltage drop across the resistor is too small to be measured practically. If a resistance value is chosen to accommodate small currents (corresponding to low volumes), then at loud volumes the voltage drop across the resistor may be large and may lead to excessive power dissipation and/or heating at the resistor. Neither of these conditions is desirable.
Accordingly, there exists a need for a mechanism in an implantable hearing aid that can alert the user when the battery charge is low. Such a mechanism may also be used in other power-monitoring applications that have a large dynamic range.